Phosphate tightly associated with protein has been known since the late nineteenth century. Since then, a variety of covalent linkages of phosphate to proteins have been found. The most common involve esterification of phosphate to serine, threonine, and tyrosine with smaller amounts being linked to lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine. The occurrence of phosphorylated proteins indicates the existence of one or more protein kinases capable of phosphorylating amino acid residues on proteins, and also of protein phosphatases capable of hydrolyzing phosphorylated amino acid residues on proteins.
Kinases play a critical role in the mechanism of intracellular signal transduction. They act on the hydroxyamino acids of target proteins to catalyze the transfer of a high energy phosphate group from adenosine triphosphate (ATP). This process is known as protein phosphorylation. Along with phosphatases, which remove phosphates from phosphorylated proteins, kinases participate in reversible protein phosphorylation. Reversible phosphorylation acts as the main strategy for regulating protein activity in eukaryotic cells.
Protein kinases play critical roles in the regulation of biochemical and morphological changes associated with cell proliferation, differentiation, growth and division (D'Urso, G. et al. (1990) Science 250: 786–791; Birchmeier. C. et al. (1993) Bioessays 15: 185–189). They serve as growth factor receptors and signal transducers and have been implicated in cellular transformation and malignancy (Hunter, T. et al. (1992) Cell 70: 375–387; Posada, J. et al. (1992) Mol. Biol. Cell 3: 583–592; Hunter, T. et al. (1994) Cell 79: 573–582). For example, protein kinases have been shown to participate in the transmission of signals from growth-factor receptors (Sturgill, T. W. et al. (1988) Nature 344: 715–718; Gomez, N. et al. (1991) Nature 353: 170–173), cell cycle progression and control of entry of cells into mitosis (Nurse, P. (1990) Nature 344: 503–508; Maller, J. L. (1991) Curr. Opin. Cell Biol. 3: 269–275) and regulation of actin bundling (Husain-Chishti, A. et al. (1988) Nature 334: 718–721).
Kinases vary widely in their selectivity and specificity of target proteins. They still may, however, comprise the largest known enzyme superfamily. Protein kinases can be divided into two main groups based on either amino acid sequence similarity or specificity for either serine/threonine or tyrosine residues. Serine/threonine specific kinases are often referred to as STKs while tyrosine specific kinases are referred to as PTKs. A small number of dual-specificity kinases are structurally like the serine/threonine-specific group. Within the broad classification, kinases can be further sub-divided into families whose members share a higher degree of catalytic domain amino acid sequence identity and also have similar biochemical properties. Most protein kinase family members also share structural features outside the kinase domain that reflect their particular cellular roles. These include regulatory domains that control kinase activity or interaction with other proteins (Hanks, S. K. et al. (1988) Science 241: 42–52).
Almost all kinases contain a catalytic domain composed of 250–300 conserved amino acids. This catalytic domain may be viewed as composed of 11 subdomains. Some of these subdomains apparently contain distinct amino acid motifs which confer specificity as a STK or PTK or both. Kinases may also contain additional amino acid sequences, usually between 5 and 100 residues, flanking or occurring within the catalytic domain. These residues apparently act to regulate kinase activity and to determine substrate specificity. (Reviewed in Hardie, G. and Hanks, S. (1995) The Protein Kinase Facts Book, Vol I:7–20 Academic Press, San Diego, Calif.).
Approximately one third of the known oncogenes encode PTKs. PTKs may occur as either transmembrane or soluble proteins. Transmembrane PTKs act as receptors for many growth factors. Interaction of a growth factor to its cognate receptor initiates the phosphorylation of specific tyrosine residues in the receptor itself as well as in certain second messenger proteins. Growth factors found to associate with such PTK receptors include epidermal growth factor, platelet-derived growth factor, fibroblast growth factor, hepatocyte growth factor, insulin and insulin-like growth factors, nerve growth factor, vascular endothelial growth factor, and macrophage colony stimulating factor.
Soluble PTKs often interact with the cytosolic domains of plasma membrane receptors. Receptors that signal through such PTKs include cytokine, hormone, and antigen-specific lymphocytic receptors. Many PTKs were identified as oncogene products by the observation that PTK activation was no longer subject to normal cellular controls. Also, increased tyrosine phosphorylation activity is often observed in cellular transformation, or oncogenesis, (Carbonneau, H. and Tonks, N. K. (1992) Annu. Rev. Cell Biol. 8:463–93.) PTK regulation may therefore be an important strategy in controlling some types of cancer.
One example of regulation of a cellular function by reversible protein phosphorylation is in the case of eukaryotic initiation factor-2 (eIF-2). When phosphorylated by an eIF-2 kinase at the alpha subunit, eIF-2 is inhibited from continuing to participate in the initiation of protein translation, which leads to termination of protein synthesis. Thus inhibitors of eIF-2 phosphorylation are expected to be anti-proliferative in nature. This regulation of eIF-2 apparently plays a role in eukaryotic cells under viral infection, nutritional deprivation, and heat shock conditions. Phosphorylation of eIF-2 apparently also plays a role in programmed cell death.
Phosphorylation of eIF-2 may be controlled by regulating the eIF-2 kinase, which is activated by double-stranded RNA (dsI). Double stranded RNA is induced by interferon and represents an interferon mediated response to viral infection. Thus inhibitors of eIF-2 phosphorylation are expected to also be anti-viral in nature.